The present invention relates to a method for manufacturing a micromechanical component, in particular, a surface-micromechanical yaw sensor, including the steps of forming a micromechanical pattern on the front side and on the back side.
Although in principle applicable to any micromechanical components, the present invention and its underlying problem are explained in greater detail with respect to this known surface-micromechanical yaw sensor.
When manufacturing micromechanical components, in addition to the front side of a wafer, for example, made of silicon, it is also possible to process the back side to make a micromechanical pattern on it. If, in the process, micromechanical patterns or layers are already located on the front side of the wafer, these patterns or layers can be damaged or contaminated by particles by putting the wafer down on an equipment holder. This can negatively influence the operability of the completed component.
German Patent No. 195 39 049 relates to a method for manufacturing a Coriolis yaw sensor having vibrating carrier masses resiliently suspended on a substrate as well as driving means for exciting a planar vibration of the carrier masses, and evaluating means for detecting a Coriolis acceleration. Provision is made for the vibrating carrier masses as well as for the driving means and integrated limit stops to be patterned from a silicon wafer in a joint operation by plasma etching.
In the specific cause of the yaw sensor which is known form German Patent No. 195 39 949, both surface- and bulk-micromechanical operations are necessary. In this context, the wafer is put down on equipment holders with the already pretreated front side for the processing on the back side. Possibly existing particle in these devices can adhere to the patterned surface of the front side and be transferred to the underlying layers during later etching steps. By the subsequent process steps, freely movable particles can be formed which constitute a reliability problem in the respective micromechanical component.
From German Patent No. 44 42 033, a yaw sensor is known, as well, during whose manufacture front-side and back-side processes have to be carried out on a semiconductor substrate.
FIG. 4 is a schematic sectional view of a known micromechanical yaw sensor which is manufactured according to a conventional method.
In FIG. 4, reference symbol 1 designates an Si-substrate, 2 a lower oxide, 3 a buried printed circuit trace made of polysilicon, 4 refers to an upper oxide, 6 to a bonding frame made of epitaxial polysilicon, 7 a bonding pad made of aluminum, 9 a seal glass, 10 an Si-protective cap, 100 an Si-wafer, 20 a vibrator, 30 a comb structure, VS a front side, and RS a back side.
In the conventional technology, micromechanical patterns are laid bare, in particular on front side VS, from 10 xcexcm thick layer 6 made of polysilicon by trenching (forming trenches) and removing the underlying sacrificial layer (oxide 2, 4).
On the back side, deep etching into Si-wafer 1 is carried out. During the application and patterning of the etching mask required for this, at least a part of the front-side patterns already exists during this etching step, and the Si-wafer must be put down on its front side. This often results in contaminations and corresponding losses in yield.
In comparison with the known design approach, the method according to the present invention has the advantage that the reliability and the operability of the micromechanical component can be significantly increased by a simple modification of the known method, namely only by adding a deposition step and an etching step, and that the particle contamination during the double-side processing of semiconductor wafers or substrates for micromechanical components can be reduced.
The basic idea of the present invention is, in the processing of micromechanical patterns, to protect the front side of a wafer or substrate by a temporary protective layer during the back-side processing, and to remove the temporary protective layer at an appropriate time so as to reduce the defect density. Thus, the core of the present invention is the introduction of a protective layer on the front side of a substrate which picks up particles during the back-side processing, and which can be removed subsequently together with the particles in a residue-free and selective manner over the already deposited layers or pattens.
According to a preferred embodiment, the protective layer is removed immediately upon the formation of the micromechanical pattern on the back side.
According to a further preferred refinement, the protective layer is left on the back side subsequent to the formation of the micromechanical pattern and used as an additional mask for the further processing of the micromechanical pattern on the front side. This has the advantage of extending the protection to later front-side processes.
According to another preferred embodiment, the protective layer is patterned on the front side.
According to a further preferred refinement, the protective layer is removed selectively by an isotropic wet- or dry-etching process. This has the advantage of an optimum lift-off behavior.